An electromotive flowmeter of a sine wave excitation scheme using a sine wave for an exciting current to be supplied to an exciting coil has a drawback susceptible to commercial frequency noise. A high-frequency excitation scheme using an exciting current with an increased frequency can solve this drawback. An electromotive flowmeter of a high-frequency excitation scheme is disclosed in, for example, JNMIHF edition, “Flow Rate Measurement A to Z for Instrumentation Engineers”, Kogyo Gijutusha, 1995, pp. 143-160 (reference 1). The high-frequency excitation scheme has a merit of being robust against 1/f noise such as electrochemical noise and spike noise and can improve responsiveness (characteristic of making a flow rate signal quickly follow up a change in flow rate).
The electromagnetic flowmeter of the sine wave excitation scheme has a structure in which a magnetic field is always changing, and the magnetic field is symmetrically distributed on the front and rear sides of the measuring tube, which are bordered on an electrode axis, in order to eliminate the influences of inter-electrode electromotive force components produced by this change in magnetic field. In practice, as the positions of electrodes and leads shift and the symmetry of the magnetic field generated from a coil deteriorates, the electromagnetic flowmeter is influenced by the components generated by temporal changes in magnetic field. The electromagnetic flowmeter of the sine wave excitation scheme therefore removes the influences of components generated by temporal changes in magnetic field as offsets at the time of calibration. However, the electromagnetic flowmeter is influenced by a magnetic field shift, a change in magnetic field distribution, and the like, and the zero point of the output of the electromagnetic flowmeter inevitably shifts. In addition, although the electromagnetic flowmeter of the sine wave excitation scheme cancels components due to changes in magnetic field by phase detection, since this phase detection is not stable, the zero point of the output is poor in stability.
An electromagnetic flowmeter of a rectangular wave excitation scheme, which uses a rectangular wave for an exciting current to be supplied to an exciting coil, uses a technique of detecting an inter-electrode electromotive force when a magnetic field stops changing, and hence is superior in the stability of the zero point of the output to the sine wave excitation scheme (see, for example, reference 1). The electromagnetic flowmeter of the rectangular wave excitation scheme cannot ignore the influences of the impedance of the exciting coil, the responsiveness of an exciting current, the responsiveness of a magnetic field, and overcurrent losses in the core of the exciting coil and the measuring tube as the frequency of the exciting current increases. This makes it difficult to maintain rectangular wave excitation (i.e., to detect an inter-electrode electromotive force in a place where no magnetic field change occurs), and makes it impossible to ensure the stability of the zero point of the output. As a consequence, in the case of the electromagnetic flowmeter of the rectangular wave excitation scheme, it is difficult to perform high-frequency excitation, and it is impossible to improve responsiveness with respect to a change in flow rate and remove 1/f noise.
Either the sine wave excitation scheme or the rectangular wave excitation scheme does not allow recognition of whether the zero point has shifted, while a fluid to be measured is kept flowing. This makes it necessary to stop the fluid to be measured so as to set the flow rate to 0, check whether the zero point of the output has shifted, and correct the offset of the set zero point.
The shift of the zero point of the output will be described with reference to FIG. 18. Referring to FIG. 18, U1 and U3 represent periods during which the flow velocity of the fluid to be measured is 0, and U2 represents a period during which the flow velocity is 1 (m/sec). Assume that, in spite of the fact that the flow velocity of a fluid to be measured has not changed, a magnitude V of the flow velocity measured by the electromagnetic flowmeter changes. In this case, the shift of the zero point can be thought of as a factor for this output variation.
Assume that the electromagnetic flowmeter is calibrated such that in an initial state, when the flow rate of a fluid to be measured is 0, an output from the electromagnetic flowmeter is 0 (v), and when the flow velocity is 1 (m/sec), the output becomes 1 (v). In this case, an output from the electromagnetic flowmeter is a voltage representing the magnitude V of a flow velocity. With this calibration, if the flow velocity of a fluid to be measured is 1 (m/sec), an output from the electromagnetic flowmeter should be 1 (v). When a given time t1 elapses, however, an output from the electromagnetic flowmeter becomes 1.5 (v) in spite of the fact that the flow velocity of the fluid to be measured remains at 1 (m/sec). Even if the flow velocity is returned to 0, 0.5 (v) may be output; the output may not become 0. The shift of the zero point can be though of as a factor for this output variation. The phenomenon of the shift of the zero point occurs as the voltage generated by a change in magnetic field varies due to a change in the ambient temperature of the electromagnetic flowmeter or the like, and the variation cannot be canceled.